At the moment structural integrity and high production rate are fundamental characteristics in the aeronautical industry, where competitiveness between airlines is very tough.
Years ago aircraft were mostly or totally built up with metallic components, providing a good performance in terms of mechanical behavior but, as a drawback, they were penalized in terms of weight.
As the aeronautical industry requires structures that, on the one hand, withstand the loads to which they are subjected, meeting high requirements of strength and stiffness, and on the other hand, are as light as possible, the use of composite materials (as carbon fiber reinforced polymers, CFRP) in primary structures is more and more extended, because with appropriate application of these composite materials it is possible to achieve an important weight saving relative to a design in metallic material.
Carbon fiber reinforced composite materials are difficult to adapt to curved surfaces, the appearance of wrinkles during their forming process being very common. The process of forming from a flat laminate to create a “C” shape is achievable as far as no significant curvature is required. The conventional process usually comprises laying up prepreg plies to form a flat laminate, a forming process to create the “C” shape, and finally a curing process. In this process, if there is a curvature in the piece, the appearance of wrinkles is very common.
Nowadays, the methods used to form the “C” profile of a spar are hot-forming (for simple “C” profiles), press-forming (for higher thicknesses and small curvature) and direct deposition with Advanced Fiber Placement (AFP) 3D technology (when the curvatures are very high; however, this technology has some restrictions).
These methods allow the manufacturing of classical C-shaped spars, but when the curvature is higher, the appearance of wrinkles does not allow the validation of the process.
EP 1775106 A1, referred to a “manufacturing method for a curved spar and other curved objects,” discloses a method for producing an object of composite material comprising the steps of:
placing one or several prepregs on a plane surface, forming a fiber stack;
lifting over the fiber stack to a molding tool,
downforming the fiber stack,
curing the downformed fiber stack, and
orienting the fiber directions so that all fibers, when the fiber stack is lifted over to the molding tool, will cross an edge of the molding tool only once.
In this document the orienting of the fiber directions of the prepregs in the fibers stack is such that none of the fiber directions of the prepregs occur parallel with a curve of the molding tool. The downforming may then be carried out in a simpler manner.
WO 2010/056164 A1, referred to a “method of forming a composite article,” discloses a method in which it is possible to hot drape form an article on a tool without creating wrinkles in the article. In this method a certain stacking sequence is used for avoiding wrinkling when forming an article by hot drape forming. Plies having fibers in the longitudinal direction of the tool, plies having fibers in the orthogonal direction of the tool and fibers having a diagonal direction of the tool are arranged in a specific order in the stack to avoid wrinkles. However, in some cases there might still be some problems during the forming in which wrinkles are formed in the article.
WO 2012/099512 A1, referred to “a composite article and a method of forming a composite article,” intends to solve the problem of the formation of wrinkles in the stack of plies during the formation process of the composite article. This document relates to a composite article, wherein the article has a longitudinal direction and a transversal direction, the article comprising a stack of plies wherein one ply is a bottom ply and one ply is a top ply, most of or all of the plies comprising fibers, and the article comprising a plurality of plies having fibers substantially in the orthogonal direction to the longitudinal direction of the article and a plurality of plies having fibers substantially in the same direction as the longitudinal direction of the article, wherein at least one of the ply/plies having fibers substantially in the orthogonal direction to the longitudinal direction of the article comprises fibers that are stiffer than the fibers in the other plies which have less stiff fibers.